Add comment to network service's pref dependency.

NOTRY=true

Bug: none
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I569974af395e6afd27ca2ef41e358704428cd01c
Reviewed-on: https://chromium-review.googlesource.com/779247


Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518265}